Configure item form so that item_code is autogenerated from naming_series (V13)

I want to set the item_code to be autonamed by default, but allow the user to override selected instances with a custom number. The item_code will form the basis of a barcode system for items.

I have run the naming series tool and I can see that this successfully configures/customises the naming series field in the item form and the Auto name is set to naming_series:

My naming series is ITM-.#####

I expect that I need to reference the naming_series variable in the item_code field, presumably under Default and Option, but this is not working. I have tried many different ways but end up with literal string or data type issues.

What is the method to insert the variable (naming_series) into anther field (item_code)

I’m sure this simple but as a beginner the answer eludes me. Support/comments would be appreciated.